Teach your dog to target things with her paw(s)---I used the end of a rubber mallett--this gave her a large target that one or both paws could easily fit on, and it was easily recognizable. I would hold a treat, and Mud would try to figure out how to earn it. She would try to shake paws, so I put my hand on the mallet, and she responded by putting her paw on my hand, on the mallet. I'd reward her, and then started removing my hand. She figured out putting her paw on top of the mallet earned her a treat. Then I started encouraging her to use both paws. From there, I replaced the mallet with a chair, sitting the mallet on the edge of the chair so she was still targeting it. Soon I took the mallet away and she would put both paws on the edge of the chair when I touched the chair with my pointer finger.
From there, I had her put her paws up on the chair, and held a treat between her front legs. Soon she understood that she was to put her front paws on the chair and bow her head. Then I added the command. Kyra Sundance's book "101 Dog Tricks" is fantastic and uses a similar method to teach this trick. If the advice I've given you isn't quite clear enough for you, I'd suggest getting her book or looking her up on the internet. She does a wonderful job and I've found her book extremely helpful and clear. =) Good luck!